1. Field of the Invention
The present invention is related to data interleaving systems used in communications systems.
2. Background Art
Multi-dimensional communications is a technique used to transmit a single high rate (speed) data stream (e.g., gigabit data speeds or rates) as multiple lower rate data streams between first and second communications devices. Each dimension is represented by symbols or bits (an, bn, cn, . . . ) in the data stream. The high rate data stream is multiplexed into multiple lower rate data streams, for example one data stream per symbol or bit, prior to transmission from the first communications device. Each of the lower rate data streams is transmitted over an independent channel. The multiple channels are often referred to as dimensions in a high order signaling scheme. The data stream having lower rate parallel data streams is received at the second communications device and is multiplexed back into a single high speed data stream to recover the transmitted data. Examples include physically independent channels, such as the multiple conductor (e.g., wire) pairs in a Cat 5 unshielded twisted pair (UTP) cable, or electrically independent channels, such as the multiple carriers in a frequency domain multiplex (FDM) system.
Forward error correction (FEC) is used to detect and correct transmission errors that can occur during transmission of the lower rate data streams. If errors occur in bursts (e.g., groups), FEC may not be able to handle the burst and may fail to correct the errors. This may be true even though the average error rate is within the correction capability of the FEC. Burst errors can occur in the transmission channel or can be the result of signal processing, such as error propagation from decision feedback equalization or the inner code of concatenated FEC.
Time domain interleaving is a technique for spreading out bursts of errors so that the peak error rate more closely approximates the average error rate. This is important because FEC algorithms are most effective when processing average error rates, but can be much less effective when processing bursts of errors. Although time domain interleaving can be used to break up the bursts, time domain interleaving can have high latency. Latency usually refers to how long a system must wait until data starts to flow, while throughput usually refers to a speed of data flow. Thus, although throughput may be high in time domain interleaving, the latency can be relatively long. The long latency can become a problem in two-way communications systems, such as telecommunications, Ethernet, Internet, Intranet, etc, which can limit applications that utilize interleaving causing degraded FEC effectiveness.
Time-domain interleaving approaches (e.g., convolutional, block, etc.) use large memories that store data in natural order and read out data in permuted order. The time-domain approach adds an inherent latency that is proportional to the interleaving depth. Also, the use of time-domain interleaving is precluded in applications that cannot tolerate large latency. Thus, latency sensitive applications, such as voice and other two-way interactive channels, cannot make use of time-domain interleavers to combat burst errors in conventional systems.
Additionally, the noise characteristics of physically independent channels in a multi-channel system may not be identical. The error rate will be bounded by the channel with the lowest signal-to-noise ratio (SNR) even when the remaining channels have higher SNR.
Therefore, what is needed is an interleaving system and method that substantially reduces latency, balances SNR of independent channels, and maintains high throughput in a communications system that uses FEC.